The Earth is approaching a dangerous threshold where it may longer be a “safe operating space” for human beings in the coming decades, according to a new study published in the journal Science by 18 researchers who try to estimate at what point the natural world reaches its breaking point.

The paper says we have crossed four of the nine “planetary boundaries”: extinction rate, deforestation, the flow of nitrogen and phosphorous into the ocean, and the level of carbon dioxide into the atmosphere, according to a Washington Post report.

Human activities are destabilizing the global environment, according to Will Steffen, the scientist who is the lead author of the paper. He holds appointments at the Australian National University and Stockholm Resilience Center, according to the Post report.

Steffen called them urgent matters, not future problems, noting that the economic boom since 1950 and a globalized economy have accelerated the pace at which humans are encroaching on these planetary boundaries. Although it’s impossible to say when the tipping point will be, the destabilization of Earth could occur in less than a century, he said.

His team focused on nine planetary boundaries that were first identified in a 2009 paper, which sets limits on changes to the environment. Beyond the boundary is a “zone of uncertainty” that shows that not much is known about what happenes when the boundaries are crossed, and gives people some time to make changes. But they represent unknown planetary conditions.

One scientist described the boundaries as an avalanche warning on a ski slope, or a high-temperature gauge on one’s car.

Human civilization has existed for 10,000 years during relatively stable environmental conditions, and no one knows exactly what will happen if these boundaries continue to be crossed, but scientists believe it is likely to at least make the planet much less hospitable to the development of humankind.

The paper did not provide solutions, but simply explained the problem.

The paper represents exploration of a new field of science known as Earth Systems Science, which draws on ecology, geology, chemistry, atmospheric science, economics, and marine biology, according to the report.

One boundary humans have exceeded if the safe-operating-zone boundary for CO2, which was estimated at 350 parts per million, a limit humans have already succeeded at 400 ppm. The zone of uncertainty for CO2 is considered to be 350 to 450 ppm. With CO2 continuing to rise at 2 ppm per year, we could surpass even that in just a couple of decades.